The invention relates to a beam landing distortion correction arrangement.
An image displayed on a cathode ray tube (CRT) can suffer from imperfections or distortions such as defocusing or nonlinearity that is incidental to the scanning of the beam on the CRT. Such imperfections or distortions occur because the distance from the electron gun of the CRT to the faceplate varies markedly as the beam is deflected, for example, in the horizontal direction. Reduction in the amount of defocusing that occurs as the beam is deflected in the horizontal direction, for example, may be obtained by developing a dynamic focus voltage having a parabolic voltage component at the horizontal rate and applying the dynamic focus voltage to a focus electrode of the CRT for dynamically varying the focus voltage.
It is known how to derive the parabolic voltage component at the horizontal rate from an S-correction voltage developed across the S-shaping capacitor of a horizontal deflection output stage. However, in a deflection circuit in which neither of the terminals of the xe2x80x9cS-shapingxe2x80x9d capacitor is at an alternating current (AC) ground potential, referred to as floating above ground level, the extraction of the parabolic voltage can be, undesirably, too complicated. Therefore, where the xe2x80x9cS-shapingxe2x80x9d capacitor is floating above ground level it can be desirable to generate a parabolic, low-level dynamic focus waveform without extracting the waveform from the voltage developed across the S-shapingxe2x80x9d capacitor. For example, a diode modulator type circuit for correcting pincushion distortion can have the xe2x80x9cS-shapingxe2x80x9d capacitor floating above the ground reference level. This makes it impractical to use the parabolic voltage waveform developed across such xe2x80x9cS-shapingxe2x80x9d capacitor for producing the dynamic focus waveform.
A television receiver, computer display or monitor can have the capability of displaying picture information on the same CRT using a deflection current at selectively different horizontal scan frequencies. In a multifrequency horizontal deflection system it can be desirable to keep the peak-to-peak voltage of the dynamic focus waveform constant when the AC waveforms in the deflection circuit are changing with different frequencies. The required amplitudes of the parabolic voltage component of the dynamic focus voltage can need to be the same at each horizontal deflection frequency. Therefore, it can be desirable to control the amplitudes of the parabolic voltage component of the dynamic focus voltage at the different horizontal frequencies. Furthermore, instead of a parabolic voltage component of the dynamic focus voltage, a xe2x80x9cbathtubxe2x80x9d shaped voltage waveform can be desirable for the newer tubes such as the xe2x80x9ctrue flatxe2x80x9d CRT""s.
A video apparatus, embodying an inventive feature, includes a parabola generator responsive to an input signal for generating a parabolic periodic signal to produce a field in a beam path of a cathode ray tube that varies in accordance with the parabolic periodic signal. An amplitude detector is responsive to the parabolic periodic signal for generating a control signal that is indicative of an amplitude of the parabolic periodic signal. A comparator responsive to a signal at a reference level and to the control signal and coupled to the parabola generator for regulating the amplitude of the parabolic periodic signal in a gain control negative feedback manner.